In work vehicles, used for working both on the fields and worksites, the laws that refer to safety require that such vehicles can be controlled even if the hydraulic units are damaged that take pressurised oil to the various driving and working apparatus on the vehicle.
To be able to keep the vehicle under control at all times, the pressurised oil, supplied by a pumping unit, is distributed by suitable hydraulic distributors that see to sending the correct quantities according to the specific operating requirements of the apparatus they are for and complying with certain sending priorities so as to be able to keep the vehicle's steering parts supplied at all times, even to the detriment of the other apparatus, so that control of the vehicles is guaranteed even under emergency conditions such as, e.g., a leak in the hydraulic circuit.
It is also necessary to underline the fact that the work vehicles have different weight and performance, i.e. in other words, they are classified in classes that are normally divided by vehicles of a lightweight class, vehicles of a medium weight class and vehicles of a heavy weight class.
The general standards that govern the construction of work vehicles require, as said above, that when a leak occurs in the hydraulic circuit that supplies the servo driving controls with pressurised oil as well as all the other working apparatus the vehicles are equipped with, it is still possible for the driver to steer for a certain length of time right after the leak occurs so the driver can drive the vehicle over to an area where it cannot generate a hazard due to the leak that has interrupted the supply of pressurized oil to the servo controls that operate the steering units, making the vehicle uncontrollable after a very short period of time.
In the case of vehicles belonging to the light weight class, the problem is felt less because, due to the relatively light weight of such vehicles, they can still be driven even under emergency conditions and without the action of servo mechanisms that are designed for this purpose and mounted on the vehicles even though the effort required of the driver is quite considerable.
In vehicles belonging to the medium weight class, hydraulic driving units are mounted that operate the steering parts of the vehicle and that are supplied with a dual displacement of oil from the pumping units: a higher displacement is supplied under normal operating conditions so steering is possible with only small turns of the steering wheel, a smaller displacement is supplied under emergency conditions to ensure the steering parts are supplied even if the steering wheel has to be turned a lot just for tiny steering manoeuvres.
In vehicles belonging to the heavy weight class, two pumping units are mounted in such a way that they are independent, one main unit and one auxiliary unit.
The main pumping unit is operated by the vehicle's engine and supplies the servo controls of the steering units and the working apparatus during normal operation of the vehicle, while the auxiliary pumping unit, which is connected to the transmission parts that transmit movement to the wheels from which it receives motion in order to be able to work, supplies small quantities of pressurised oil under emergency conditions so that, when there is a leak, the vehicle continues moving by inertia transmitting movement to the auxiliary pumping unit which, in turn, is able to supply a sufficient quantity of oil to allow a few steering manoeuvres before the vehicle stops once and for all.
This state of the art has some drawbacks.
A first drawback concerns the medium vehicle class: the dual displacement steering units are very expensive and influence the overall cost of the vehicles.
Besides this, under emergency conditions, these dual displacement steering units, while they do allow steering also under critical conditions they require, in order to do so, a lot of turning of the steering wheel with considerable physical effort by the driver.
In addition, these dual displacement pumping units are able to solve the problem of supplying with oil to the steering servo controls of vehicles in the medium class only, since their structure is such that it cannot operate the driving servo controls of vehicles in the heavy class under emergency conditions.
A second drawback appertaining to vehicles belonging to the heavy class is that the auxiliary pumping units are also very expensive and are an additional element that has to be mounted on the vehicles.
In addition, these secondary pumping units, as they are continuously operated by the movement of the wheels also when they are not required, continuously absorb a considerable amount of engine power which, however, in the greatest part of the vehicles' use cycle, is not used.
In addition to the above, if the pumping parts are set to supply several working apparatus mounted on the vehicle and they require different oil pressures, it could happen that the servo controls that operate the steering parts are sent oil at a pressure that is too high which could damage them: thus, it is necessary to mount an additional pressure compensating device that limits the pressure of the oil sent to the servo controls that operate the steering parts, e.g. by reducing the flow of oil.